This invention concerns precipitated calcium carbonate. More specifically, it concerns a novel method for precipitating a calcite of minute particle size and high purity by the carbonation of an aqueous lime slurry in the presence of certain organophosphorous compounds.
Precipitated calcium carbonate is commonly prepared by the carbonation of aqueous lime slurries, and numerous agents, such as those disclosed in U.S. Pat. Nos. 2,188,663, 2,242,228, 2,467,082, 3,003,010, 3,126,253, 3,133,824, 3,347,624, 3,443,890 and 4,018,877, Japanese Patent Publication No. 71-7373 and Patent Disclosures Nos. 48-103100 and 53-53598, and USSR Pat. No. 323365, have been proposed to control the particle size of the precipitate. Such agents, however, have apparently failed to produce a calcite of high purity having a particle size in the order of 0.01 micron. It is therefore a primary objective of the present invention to develop an economic process for the preparation of a precipitated calcite of such purity and particle size by the carbonation of aqueous lime slurry.
Carbonation of lime slurries at starting temperatures below 20.degree. C. is disclosed in such references as U.S. Pat. Nos. 1,654,099, 2,141,458, 2,188,663, 2,631,922, 3,126,253, 3,133,824, 3,320,026 and 4,133,894 and in Japanese Patent Disclosure No. 48-103100, but, again, such preparations have apparently failed to produce the presently sought calcite.
Use of polyphosphonates as detergent builders or as dispersants in systems containing calcium carbonate is disclosed in U.S. Pat. Nos. 3,925,228 and 4,049,586 and in Belgium Pat. No. 855,456. The influence of these phosphonates on the nucleation rate and crystal growth of calcium carbonate from supersaturated solutions is disclosed by Reddy and Nancollas in "The Effect of Phosphate and Phosphonates on Nucleation and Crystal Growth of Calcium Carbonate," Prepared Papers, National Meeting, Dir. Water, Air and Waste Chem., Amer. Chem. Soc., 12, No. 2, 212-16, August, 1972 and in "Calcite Cyrstal Growth Inhibition by Phosphonates," Desalination, 12, 61-73, 1973; use of the phosphonates for the production of precipitated calcium carbonate by carbonation of lime slurry is not, however, contemplated by these publications.